Secrets of the penguin waddle

Related Stories

Penguins are notoriously inefficient walkers, expending twice as much energy as other animals of the same weight but new research has found that it's not because they waddle from side to side - the real problem is their short legs.

Correspondence in this week's Nature by two US researchers shows that waddling actually helps penguins conserve energy.

"Our findings indicate that walking is expensive for penguins ... because they have such short legs that require their leg muscles to generate force very quickly when they walk," said student biology researcher Timothy Griffin.

"Our hunch was that if penguins are trying to move forward, but expend energy rocking side to side with this awkward, roly-poly, back-and-forth movement, then it's got to be wasted energy," said co-researcher Rodger Kram, of the University of Colorado, Boulder. "But what we found is that they are inefficient because of their short legs and big feet, and waddling is a means to cut their losses."

The waddling motion is analogous to an inverted pendulum swinging rhythmically back and forth, Griffin explained. At the end of each swing, when the penguin is momentarily still, the energy of side-to-side motion is stored as potential energy.

On the return swing, this is converted efficiently into energy of motion that peaks as the penguin rocks back through the vertical. The energy again is stored as potential energy as the penguin comes to a stop at the opposite extreme. The penguins also rock fore and aft as they move forward, employing the same inverted pendulum motion used by all animals, even four-legged animals.

Griffin and Kram performed their experiments on Emperor penguins in a large refrigerated penguin city where they nudged naturally curious penguins across a platform which enabled them to measure the various forces involved in penguin walking.

The team calculated that, without the side to side motion, penguins would be less efficient. "The penguin's rocking motion helps raise their centre of mass," said Griffin. "Without it, their muscles would have to make up that work."

Some penguins walk over 160 kilometres from their rookeries to the open sea after fasting for four months during the harsh Antarctic winter so it would seem strange they have such an inefficient walk.

Evidently, Griffin said, penguins have made an evolutionary trade-off. Their short legs make them more streamlined swimmers and divers, even if their walking is less efficient. Short legs may also help reduce heat loss, especially while incubating the eggs in winter.

Griffin and Kram's work has implications not only for the ecology and evolution of penguins, but for the improved understanding, evaluation and treatment of individuals with gait disabilities, and maybe waddling in pregnant women.